This research is focussed on two fundamental and widespread forms of behavioral plasticity; choice, which is governed by behavioral hierarchies, and associative learning. Both phenomena are being studied in the marine gastropod mollusk Pleuro-branchaea californica. Behavioral hierarchies are the means by which unrelated acts of behavior are organized into a "priority sequence" such that under specific ciicumstances, the "correct" behavior pattern is performed to the exclusion of others. Experiments on Pleurobranchaea are being performed to determine the neuro-physiological and hormonal mechanisms underlying choice. Associative learning has been demonstrated by means of classical conditioning and avoidance conditoning of whole animals and isolated nervous systems. Neurophysiological substrates of such conditioning are being sought by microelectrode studies on "command" neurons that control the conditioned behavior (feeding).